1. Field of the Invention
The present invention relates to a LED driver circuit, and more particularly to a LED driver circuit capable of providing a regulated load current and a TRIAC holding current for dimmable LED lighting applications.
2. Description of the Related Art
In a LED lighting application utilizing an AC power source, the load current for a LED module has to be regulated to not only provide a stable lighting but also protect the LED module.
FIG. 1 illustrates a prior art LED driver circuit having a simple architecture. As illustrated in FIG. 1, the LED driver circuit includes a resister 110 and a LED module 120, and a line voltage VIN is applied to provide a load current IL. The resistance of the resistor 110 is determined according to the maximum of the line voltage VIN, and both the minimum voltage dropt and the maximum rated current of the LEDs in the LED module 120 as well, to protect the LED module. However, there are two disadvantages in this prior art LED driver circuit. First, the load current IL is varied with the line voltage VIN. Second, to provide better illumination, all the LEDs in the LED module 120 have to be specially sorted to have a centralized load current distribution of which the mean is close to the maximum rated current, and this will increase the cost.
One solution to make the load current independent of the line voltage in the LED lighting applications is utilizing a switching-power-supply architecture. Please refer to FIG. 2, which illustrates a prior art LED driver circuit including a LED driver controller. As illustrated in FIG. 2, the LED driver circuit includes a LED driver controller 200, an NMOS transistor 201, an inductor 202, a diode 203, a resistor 210, and a LED module 220.
The LED driver controller 200 is used to generate a gating signal VG to drive the NMOS transistor 201 in response to a current sensing signal VCS, to regulate the current sensing signal VCS, and thereby the current of the LED module 220. However, this architecture is not appropriate for small power applications due to cost issue—the LED driver controller 200 and the inductor 202 are expensive for small power LED lighting applications.
To reduce the cost for small power LED lighting applications, one solution is using a LDO (Low Dropout) regulator. Please refer to FIG. 3, which illustrates a prior art LED driver circuit utilizing a LDO regulator. As illustrated in FIG. 3, the circuit includes a LDO regulator 300, a resistor 310, and a LED module 320. The LDO regulator 300 is in series with the LED module 320 to generate a regulated output voltage VO independent of a line voltage VIN, and thereby a regulated load current IL. However, as the line voltage VIN is a high voltage—for example 220V, the LDO regulator 300 has to be a high-voltage product manufactured by a high-voltage process, which is more expensive than a low-voltage process.
In view of the problems mentioned above, the present invention proposes a novel LED driver circuit with low-cost components to regulate the load current.